Primer composition for powder coating

ABSTRACT

An emulsion type primer for powder coating, which is pollution-free and excellent in safety, achieves a good adhesion between a metallic material and a synthetic resin coating and contains water as a medium, and a process for producing a metallic material coated with a synthetic resin by using this primer. 
     This primer comprises an acid-modified diene polymer, an organic amine or ammonia and water. 
     A metallic material coated with a synthetic resin is produced by applying the primer composition onto the metallic surface, curing the same by baking and then coating it with the synthetic resin by the fluidized bed dip coating method or the electrostatic coating method.

FIELD OF THE INVENTION

This invention relates to a process for producing a metallic materialcoated with a synthetic resin, which is not only excellent in decorativecoating, corrosion protection, electric insulation, wear resistance andweatherability but also highly improved in the adhesion between themetallic material and the coating, by coating the surface of a metallicmaterial in the form of a pipe, a filament, a rod, a plate, etc. with asynthetic resin such as a thermoplastic resin or a thermosetting resinvia a specific primer. The present invention also relates to a primercomposition for powder coating.

RELATED ART

As primers for powder coating with thermoplastic powders, in particular,nylon powders, there have been already employed compounds obtained bypolymerizing diene compounds such as polybutadiene. After being appliedand dried, these primers are baked in a high-temperature oven.Accordingly, there have been employed primers containing halogenatedhydrocarbons such as trichloroethane as a medium from the viewpoints ofdryability and fire prevention. To meet the demands of the times toprotect the ozone layer from the standpoint of the global environment,however, there arises a trend of restrictions on the use of halogenatedhydrocarbons such as trichloroethane and chlorofluorocarbons. Althoughit has been proposed to use lower aliphatic or aromatic hydrocarbons assubstitutes therefor, these compounds suffer from a number of problemsin disaster prevention and working environments, for example, a lowflash point and a toxic effect on the human body.

The present inventor has invented a primer which contains water as amedium and is superior in adhesion to the conventional products bymixing a partly acid-modified diene polymer with an organic amine orammonia. Because of the use of water as a medium, this primer can solvethe problems relating to disaster prevention and working environmentswith a single stroke.

DISCLOSURE OF THE INVENTION

Accordingly, the present invention provides a primer composition forpowder coating, which contains water as a medium, prepared by mixing anacid-modified diene polymer with an organic amine or ammonia and aprocess for producing a synthetic resin-coated metallic materialcharacterized by applying the above-mentioned primer composition forpowder coating onto a metallic surface, curing and baking the same andthen coating it with a synthetic resin by a fluidized bed dip coatingmethod or the electrostatic coating method. The term "acid-modifieddiene polymer" as used herein means a polymer obtained by copolymerizinga compound having a conjugated double bond (for example, butadiene orisoprene) with a compound such as acrylic acid, methacrylic acid ormaleic anhydride or polymerizing a compound having a conjugated doublebond and then treating the polymer thus obtained with, for example,maleic anhydride.

In other words, the present invention relates to a primer compositionfor powder coating which comprises an acid-modified diene polymer, anorganic amine or ammonia and water or a process for producing a metallicmaterial coated with a synthetic resin, which involves the step ofapplying the above-mentioned primer composition for powder coating ontothe metallic surface, the step of curing the same by heating, and thestep of coating it with a synthetic resin by a fluidized bed dip coatingmethod or an electrostatic coating method.

It is preferable to mix the components at a ratio of from 5 to 50 partsby weight (still preferably from 15 to 30 parts) of the organic amine orammonia and from 300 to 1,500 parts by weight (still preferably from 400to 900 parts) of water per 100 parts by weight of the acid-modifieddiene polymer. When the mixing ratio lies within the range as definedabove, the present invention can be preferably embodied whileconsidering stable dispersion, shortening of drying time and anappropriate concentration of solid matter.

In the present invention, the carboxylic acid unit is introduced intothe polymer in order to disperse the diene polymer in water. It isdesirable that the carboxylic acid is introduced in such an amount as togive an acid value of from 5 to 500 mgKOH/g. When the acid value islower than 5 mgKOH/g, the salt, which has been formed by mixing thepolymer with an amine, shows only a low solubility in water and thus nostable aqueous solution can be formed. On the other hand, it isdifficult to prepare an acid-modified diene polymer having an acid valueexceeding 500 mgKOH/g. In such a case, furthermore, the primer coatinglayer has an unnecessarily high hydrophilic nature, which deterioratesthe adhesion to the synthetic resin coating layer, in particular, afterthe treatment with hot water.

The acid-modified diene polymer to be used in the present invention hasa molecular weight of at least 500, preferably from about 1,000 to5,000. When the molecular weight of this polymer is excessively low,only an insufficient adhesion can be achieved. On the other hand, anexcessively high molecular weight makes the resulting solution highlyviscous, which brings about a fear that the application ratio wouldexceed the appropriate level. The appropriate ratio of the applicationof the acid-modified diene polymer to an iron plate ranges from about0.2 to 1.2 mg/cm², preferably from 0.3 to 0.8 mg/cm².

In the present invention, the organic amine is used to form an aminesalt together with the carboxyl group in the acid-modified dienepolymer, and to thereby give water-solubility. The salt thus formeddecomposes in the step of baking and thus forms the original organicamine again, and then eliminated from the system. Therefore, the organicamine to be used in the present invention is not particularlyrestricted, so long as it is a commercially available organic aminehaving a low molecular weight. Examples thereof include allylamines,isopropylamines, ethylamines, isobutylamines, n-butylamines,sec-butylamines, t-butyalamines, propylamines, 2-ethylhexylamines,morpholines, pyridines, piperidines, piperazines, amino alcohols, andanilines. In addition, ammonia can be used therefor.

The primer of the present invention, which contains water as a medium,may further contain other components. For example, ethyl cellosolve,butyl cellosolve, etc. may be used therein as an auxiliary for improvingthe smoothness and fluidity of a coating film which is formed byapplying the primer onto a metal. Also, a small amount of awater-soluble rust preventive may be added to prevent excessive rusting.

It is also possible to add cobalt or manganese ions to the primersolution for acceleration of the oxidation in the baking step. It isadvantageous that the cobalt or manganese ions are added in the form ofa compound which can be uniformly dissolved in the primer solutioncontaining water as a medium (for example, cobalt naphthenate or cobaltoctylate). It is appropriate to add the cobalt and manganese ions insuch an amount as to give a total content of from 0.05 to 1.00 part byweight, preferably from 0.2 to 0.6 part by weight, per 100 parts byweight of the acid-modified diene polymer.

The production of synthetic resin-coated metallic material by the use ofthe primer composition for powder coating of the present invention iscarried out as follows; The primer is uniformly applied onto a metal,which has been optionally degreased and washed, by a conventional methodsuch as dipping. Next, the primer is cured and baked onto the surface ofthe metallic material with the use of a heating means such asradiofrequency induction heating or an electric furnace. Then it isfurther coated with a synthetic resin by the fluidized bed dip coatingmethod or the electrostatic coating method.

In the baking step, the primer thus applied is oxidized by oxygen in theatmosphere and thus gives active groups. When the baking is effectedunder mild conditions, these active groups are insufficiently formed.When the baking conditions are excessively severe, on the other hand,carbonization proceeds and thus the adhesive fore of the primer layerdecreases. Therefore the baking temperature appropriately ranges from250 to 400° C. The baking time varies depending on the bakingtemperature. For example, it is appropriate to bake the primer at 350°C. for 4 to 10 minutes.

As the synthetic resin to be used as a coating in the present invention,use can be made of a thermoplastic resin or a thermosetting resin.Particularly preferable examples thereof include polyamide,polyethylene, polyester, polyvinyl chloride and fluororesins.

As the metal to be used in the present invention, we can use variousmetallic materials for industrial use such as iron, steel and aluminum.It may be in various forms including a pipe, a filament, a rod, a plate,etc.

Because of the use of water as a medium, the primer composition forpowder coating of the present invention is free from various globalenvironmental problems (for example, the depletion of the ozone layer)encountered in the use of halogenated hydrocarbons such astrichloroethane and chlorofluorocarbons as a medium or various problemsrelating to disaster prevention and working environments encountered inthe use of lower aliphatic or aromatic hydrocarbons as a medium. Thus ametallic material coated with a synthetic resin, which is superior inadhesion to the conventional ones, can be safely produced withoutcausing any environmental pollution.

EXAMPLES

To further illustrate the present invention in greater detail, thefollowing Examples will be given.

Examples 1 to 7

200 g of liquid polybutadiene M2000-20 (molecular weight: about 2,000,acid value: 20 mgKOH/g, mfd. by Nippon Petrochemical Co., Ltd.) wasweighed and thoroughly mixed with 6.0 g of triethylamine, 60 ml of butylcellosolve and 6.0 g of cobalt naphthenate. Then 800 ml of water wasadded thereto in small portions and thoroughly mixed to thereby give aprimer wherein the mixture was completely dispersed in water (Example6). Table 1 summarizes the composition ratios of the primers in otherExamples.

An iron plate (72 mm×90 mm×3 mm in thickness) was subjected to rustremoval and degreasing. Then each primer was applied onto this ironplate by the dipping method and dried at room temperature for about 10minutes. Next, it was baked by heating in an electric furnace at 350° C.for 7 minutes and then dipped in a fluidized bed filled with a nylon 12powder (Diamid Z2073) for several seconds. After being cooled in the airfor 1 minute, it was cooled in water for several minutes.

The nylon-coated iron plate thus obtained was subjected to a hot-waterpeel test wherein it was immersed in hot-water at 95° C. for a givenperiod of time and then the coating was peeled from iron plate with aknife. Each experiment was repeated 3 or 4 times and the results weretotally evaluated. Table 1 shows the test results.

Comparative Example 1

50 g of liquid polybutadiene C-1000 (acid value, 57 mgKOH/g; molecularweight, about 1,500; mfd. by Nippon Soda Co., Ltd.) was dissolved in 50ml of cyclohexane. Further, 10 g of a surfactant Tween 60 was added andthe mixture was uniformly mixed. Then water was added while the mixturebeing stirred to produce an emulsion. After adding water so as toproduce a total volume of 550 ml, the mixture was slowly heated for thedistillation of cyclohexane.

The above-mentioned emulsion was applied onto an iron plate, which hadbeen subjected to rust removal and degreasing in the same manner as thatemployed in the above Examples, dried and subjected to the heattreatment. Then it was dipped in a fluidized bed in the same manner asthat employed in the above Examples and the nylon-coated iron plate thusobtained was subjected to the hot-water peel test. Table 1 shows theresults.

Comparative Example 2

An iron plate was treated in the same manner as that employed in theabove Examples, except using F-3 primer (trichloroethane solution, mfd.by Daicel Huels Ltd.) and dipped in a fluidized bed. The nylon-coatediron plate thus obtained was subjected to the hot-water peel test. Table1 shows the results.

                                      TABLE 1    __________________________________________________________________________    Acid-modified polybutadiene   Cobalt    (100 parts)         Triethyl                             Butyl                                  napthe-                                      Solid                                          Average                                               Hot water peel    Molecular           amine                             cellosolve                                  nate                                      matter                                          adhesion                                               test (days)    weight   Acid value (mgKOH/g)                        part part part                                      %   mg   1 3 7 14    __________________________________________________________________________    Ex. 1        1500 130        8.0  30   --  20  65   ∘                                                 ∘                                                   Δ                                                     x    Ex. 2        1500 130        8.0  30   3.0 20  70   ∘                                                 ∘                                                   ∘                                                     ∘    Ex. 3        1000 70         8.0  30   3.0 20  64   ∘                                                 ∘                                                   ∘                                                     ∘    Ex. 4        1000 20         8.0  30   3.0 20  42   ∘                                                 ∘                                                   ∘                                                     ∘    Ex. 5        2000 80         8.0  30   3.0 20  94   ∘                                                 ∘                                                   ∘                                                     Δ    Ex. 6        2000 20         8.0  30   3.0 20  53   ∘                                                 ∘                                                   ∘                                                     ∘    Ex. 7        1500 64         8.0  30   3.0 20  72   ∘                                                 ∘                                                   Δ                                                     Δ    Comp.        1500 57         aqueous emulsion surfactant                                      10  60   ∘                                                 Δ                                                   x --    Ex. 1               Tween 20, 2 wt. %    Comp.        3000 --         trichloroethane                                      10  93   ∘                                                 ∘                                                   Δ                                                     x    Ex. 2    __________________________________________________________________________     ∘: not peeled.     Δ: partly peeled.     x: peeled.

I claim:
 1. A process for producing a metallic material coated with asynthetic resin which comprises the steps of:(a) applying a coating of aprimer composition directly onto a metallic surface, (b) curing thecoating by heating, and (c) coating the cured primer with a syntheticresin by either a fluidized bed dip coating method or an electrostaticcoating method;wherein, the primer composition comprises 100 parts byweight of an acid-modified diene polymer, from 5 to 50 parts by weightof an organic amine or ammonia, and from 300 to 1,500 parts by weight ofwater; and wherein the coating is cured in step (b) of the process at atemperature of 250 to 400° Centigrade.